In commonly assigned published Italian Patent No 1,227,559, there is described a system for combining at least two signals which are received at different locations in space (spatial diversity reception) and/or at different angular orientations (angular diversity reception); FIG. 1 of that publication is a generalized schematic of such a diversity combining system. In the known diversity receiver, an intermediate-frequency estimated BER (bit err rate) function is monitored in order to provide a minimum-BER combiner.
The intermediate frequency estimated BER function is called "analog BER" and is given by the formula: EQU BER=10.sup..alpha.P+.beta. +10.sup..gamma.D+.delta. ( 1)
In this formula the variables P and D correspond to the power and dispersion of the combined signal, while the coefficients .alpha., .beta., .gamma., .delta. depend on the type of modulator-demodulator employed.
It is possible to re-express equation (1) in the form: EQU BER=A.Rag(P)+B.Ban(D) (2)
where:
Rag (P)=10.sup..alpha.P PA1 Ban(D)=10.sup..gamma.D PA1 A=10.sup..beta. PA1 B=10.sub..delta. PA1 BER(.phi.,T1,T2)
in which the function Rag(P) represents the power of the recombined data spectrum, and the function Ban(D) represents the dispersion of said spectrum.
Since power and dispersion depend on the recombination phase .phi. and the attenuation T1, T2 introduced on each of the two channels by respective attenuators, the analog BER function for any particular combination of input signals on the two channels can also be expressed as:
it being understood that the analog BER function is also dependent on the characteristics of the two input signals being applied to the combiner.
The channel that both phase-shifts and attenuates the signal upstream of the summing node may be designated MAIN and the channel that only attenuates upstream of the node may be designated DIV, as shown in FIG. 6 of the above cited Italian Patent 1,227,559 herein reproduced as FIG. 1. In this FIG the references indicate respectively:
MAIN: channel that phase-shifts and attenuates the signal upstream of the summing node. PA0 DIV : channel that attenuates upstream of the summing node. PA0 8: driven attenuator T1 PA0 9: driven attenuator T2 PA0 10: delay line PA0 11: driven phase shift PA0 12: summing node PA0 13: IF amplifier PA0 14: power detection filter PA0 15: detector PA0 16: automatic gain control PA0 17: dispersion measurement network PA0 18: A/D converter PA0 19: A/D converter PA0 20: microprocessor PA0 21: D/A converter PA0 22: D/A converter PA0 23: D/A converter
Assuming a dispersion on the MAIN channel characterized by an echo delay .tau., a notch depth B.sub.cm and a notch frequency position F.sub.nm, and also assuming a dispersion on the DIV channel characterized by an echo delay .tau., a notch depth B.sub.cd and notch frequency position F.sub.nd, these parameters will define the selective fading present on the MAIN and on the DIV channels, resulting in a first analog BER function BER.sub.1 (.phi.,T1,T2).
As the parameters determining the selective fading present on the two input channels change, one will obtain other analog BER functions BER.sub.n (.phi.,T1,T2) different from BER.sub.1 (.phi.,T1,T2). Therefore it will be possible to define countless BER.sub.n (.phi.,T1,T2)'s corresponding to the countless possible selective fading conditions.
Setting T1 and T2 equal to the same nominal attenuation value (T1.sub.n indicates a nominal value of T1 and T2.sub.n indicates a nominal value of T2) results in the function: EQU BER.sub.n (.phi.)=A.Rag.sub.n (.phi.)+B.Ban.sub.n (.phi.) (3)
For optimizing BER.sub.n (.phi.), varying .phi. in accordance with a conventional gradient search technique is usually sufficient to locate the desired minimum, assuming that the BER.sub.n (.phi.) function was reasonably smooth. However, a BER.sub.n (.phi.) which remains constant over a relatively large phase interval is not necessarily suitable for determining the optimum phase for combining the two input signals. Moreover, in general the BER.sub.n (.phi.) function will not have only one minimum but will have both a relative minimum M.sub.rel and an absolute minimum M.sub.ass (FIG. 3A); there are BER.sub.n (.phi.)'s that have a relative minimum corresponding to an "out of order" radio link, and an absolute minimum for which the link performs satisfactorily.
It should also be noted that when one tries to minimize dispersion through phase shifting, there is the risk of drastically lowering the power.
Moreover, where the recombined data spectrum presents a linear dispersion resulting from different flat attenuation or even in which the flat attenuation level of the two signals is equal, there are cases where the optimum phase for BER.sub.n (.phi.) does not result in minimize dispersion.
Different propagation features characteristic of the two channels, or a temporary mispointing of the antennas (in a spatial diversity system) can cause a level difference between the two channels which in turn may result in a degraded tolerance of the combiner to selective fading.